The present invention relates to an improved method of mooring floating offshore structures such as semi-submersible drilling rigs. Specifically, the invention relates to a method and apparatus for deploying additional mooring lines to secure a rig in anticipation of adverse weather conditions.
Floating offshore structures, for example, semi-submersible drilling platforms, drill ships and the like, are used by the oil and gas industry in increasing water depths. Unlike platforms that sit on the seabed, these structures must have a means of keeping the structure within reasonable proximity of the well site; such a means can include a mooring system and/or a dynamic positioning system. While applicable to all types of floating offshore structures, the mooring system will be explained in terms of semi-submersible drilling platforms.
Semi-submersible drilling platforms are specialized vessels used to prospect for and produce oil and natural gas from the sea floor, and are especially useful when the water depth precludes the use of fixed structures on the ocean bottom. These rigs are very large and very expensive to construct, commonly costing hundreds of millions of dollars each.
As seen in the example of FIG. 1A, semi-submersible drilling platforms comprise three main parts: buoyant members 108, work section 109, and stability columns 111, which elevate work section 109 above the water level. Work section 109 contains or supports the equipment that is necessary for drilling and other related activities, as well as the living quarters for the crew. The platform is towed or proceeds under its own power between locations with buoyant members 108 floating on the surface, but once on location, the buoyant members are filled with seawater until the platform is partially submerged with the operating water level approximately mid way up the stability columns. In this example, a drilling derrick 102 and cranes 106 for handling large items are mounted on upper deck 104 of the work section 109. For the sake of simplicity, it will be understood that this drawing is not drawn to scale, nor does it begin to show the variety of equipment carried on such a platform. Mooring lines carried on the semi-submersible drilling platform are shown deployed. These mooring lines typically consist of a length of chain, which is attached at one end to the anchor and at the opposite end to a steel cable (also referred to as wire rope). The cable is attached to a traction winch, which provides necessary tension, and is stored on a remotely located storage reel. The chain in the mooring line provides weight near the anchor, so that force applied to the anchor is always in a horizontal direction. Cable, on the other hand, provides a better strength to weight ratio, so that the strength of the line is not all used in simply supporting the length of line in the water.
FIG. 1B is a close-up of the onboard hardware for the mooring system. Chain for the anchor is carried in chain lockers 116, while cable is carried on a spool 122. A combination traction winch/windlass 120 is positioned on upper deck 104 to enable handling of the cable and chain. Traction winch/windlass 120 has three major components, some of which are better seen in FIG. 1C, which shows a side of the semi-submersible adjacent the side seen in FIG. 1A. The three components are traction winch 120A, which handles and tensions the wire rope 118, windlass 120B, which handles and tensions the chain 114, and shaft 120C, which, combined with an electric motor (not shown), provides the drive power to run the other two components. Anchors 110 are normally carried by the semi-submersible drilling platform already attached to chain 114 and positioned on structures known as anchor bolsters 124. From its attachment to the anchor 110, chain 114 is led under fairlead 112, then over the windlass 120B to the chain locker 116, with the chain and windlass holding the anchors in place. Each rig will be equipped with enough chain and cable to handle a given depth of water. In this example, the semi-submersible drilling platform carries enough chain and cable for water depths up to approximately 5,000 feet, although this value can vary according to design. Platform 175 is located on the stability column below the traction winch and provides a location and facilities for connecting the chain to the wire when the chain has been run out and the anchor line is transferred from the windlass to the traction winch. These systems will be explained in greater detail, shortly.
Depending on the water depth and the operator""s choice, the platform can be moored either by using the system that is carried onboard, or by utilizing pre-set anchors, such as suction pile anchors or deep embedded anchors connected to mooring lines comprising some combination of chain, wire rope and synthetic rope and with the free end supported by a floating buoy. In water depths greater than the platform is equipped for (5,000 feet in our example), pre-set anchors must be utilized, but in shallower water, either system can be used. For example, the operator may want to begin drilling as soon after the arrival of the rig as possible. In this instance, they may choose preset anchors, as much of the time necessary to set the anchors can take place before the rig arrives on site.
FIG. 2 shows an exemplary deployment of the mooring system of a drill platform, as seen from a birds-eye view. Drill rig 102 and cranes 106 are seen, as are traction winch/windlasses 120. Each comer has two combination traction winch/windlasses 120 run from a single shaft through clutches, with their mooring lines generally placed 45 degrees from each other and arranged symmetrically around the platform. However, it should be understood that this is only one possible arrangement of mooring lines, and even this example may be biased to resist environmental forces, such as wind and waves, which may be stronger from one direction than from another.
Setting the onboard anchor is demonstrated with reference to FIGS. 3A and 3B. A chaser line 310, attached to anchor/chain 110/114, is given to an anchor handling boat 300. This chaser line 310 encircles the anchor and chain, and as boat 300 moves away from platform 100, anchor 110 is freed from its bolster 124. The boat steams away from the rig until sufficient chain 114 has been deployed. The anchor chain 114A is then separated at platform 175 from the chain 114B in chain locker 116 and attached to the connector 176 on the free end of the wire rope running over the traction winch. One method of handling this change from chain to cable is shown in U.S. Pat. No. 3,967,572 to Santa Fe International Corporation, titled xe2x80x9cAnchoring System and Chain Stopper Thereforxe2x80x9d, which is hereby incorporated by reference. After the anchor chain 114A is attached to wire rope 118 at connector 176, boat 300 continues to steam away from the rig until the traction winch pays out the required amount of wire. The boat then lowers anchor 110 on chaser wire 310 until it reaches the seabed; then the rig tensions up the mooring line with the traction winch 120A to set the anchor 110. In FIG. 3B, the boat returns the chaser and chaser line 310 to the rig.
FIG. 4 demonstrates the conventional way of connecting to a preset anchor. Preset anchor 420 has been previously placed in position, with a mooring line 425 of some combination of chain, wire rope, and synthetic rope. Buoy 410, is attached to the mooring line to mark the location of the anchor and to support the free end of the mooring line. When platform 100 is to be attached to preset anchors, onboard anchor 110 must first be removed from the chain 114 and stored and the chain removed from the fairlead 112. Then wire rope 118 is led from traction winch 120A through fairlead 112 and given to an anchor handling boat 300. The boat runs the end of the wire out to the buoy and attaches wire rope 118 to the mooring line 325 on anchor 320. Once these are connected, the traction winch 120A takes up the slack.
As semi-submersible drilling units become larger, environmental loads on the units increase and the size of mooring lines and anchors must increase also. To offset this tendency, multiple thrusters (not shown) can relieve peak loads during storms, so that normal mooring components can be kept to a reasonable size.
While the mooring systems described above are capable of maintaining the platform""s position under normal conditions, the platform can be subjected to intense weather conditions in those parts of the world that are subject to tropical revolving storms (TRS), such as the Gulf of Mexico. It is not practical to reposition the rigs to avoid such storms, nor is it possible to utilize thruster systems if the operating policy is for personnel to be evacuated ahead of major storms. In the past, the risk of losing the rig during the storm (hurricane) was mitigated by the fact that no personnel were on board and human life was not endangered. Today, however, rigs have become larger as drilling moves into deeper water and the risk of a mooring failure has increased, due to both the increase in water depth and the larger environmental loads. The complexity and cost of the infrastructure necessary to operate in deep water has also increased, so there is a need to devise a mooring system that will prevent a rig from breaking free in a storm and from dragging an anchor and destroying nearby support and/or production structures. One solution would be to make the normal mooring systems stronger, so that they could withstand the environmental forces generated by tropical revolving storms. However, not only would the onboard equipment have to be made considerably bigger, stronger and more expensive to handle these forces, but related machinery, such as the anchor-handling boats, would also have to be made larger to handle larger anchors and to pull out heavier chain and wire. Economically, this is not a desired solution. To minimize costs to the operator, the need exists for means to economically supplement a conventionally sized mooring system to provide additional support during the season for tropical revolving storms.
The present invention relates to an improved mooring system in which additional mooring lines can be deployed without the need for a complete set of additional mooring winches. Existing traction winch/windlass devices have the capability to handle both wire and chain mooring lines, but once the anchors are deployed, only the traction winch is utilized to apply tension to the mooring lines. In implementing an embodiment of the present invention, additional fairleads are added to the mooring system to increase the capacity of the mooring system, while the specifications for the combination traction winch/windlasses are increased so that this equipment can handle the simultaneous load of a mooring line on the traction winch and a mooring line on the windlass. Using these additional capabilities, the normal mooring lines connected to the traction winches can be supplemented in hurricane season by additional mooring lines, which are run over the windlass. This solution does not require the heavier cable and chain of other suggested solutions, nor does it require heavier support boats to implement. Other than the increase in the required strength of the traction winch/windlass frame, this solution only requires that one additional fairlead be added adjacent each of the existing fairleads.